Mild steel strip of the type produced by hot rolling generally has widths of 4 to 6 ft (1.2 to 1.8 m) and continuous lengths of 2000 ft (600 m) or so. The thickness will depend largely upon the final intended use, for example for pressing into vehicle body panels. The hot rolling process has the inevitable consequence of generating an oxide coating (rust) on the freshly formed surface of the strip, and therefore after leaving the rolling mill the strip is pickled. Pickling involves passing the strip continuously through a series of baths (generally four baths) containing hot (85.degree. C.) hydrochloric acid and generally ferrous chloride, the acid strength increasing through the baths to about 10%. The acid from the final bath is squeegeed off and the strip is then rinsed, first in cold demineralized water and finally in hot demineralized water which serves to heat the steel and promote evaporation of residual water from its surface. Rinsing generally takes the form of spraying the strip with water from above and from below as it passes through the rinse section of the pickle line.
In order to maintain a degree of continuity in the pickle line, it is usual to weld the trailing end of the strip in the pickle line to the leading end of the next strip to be pickled. This welding process necessitates bringing the strip to a stop in the pickle line and in the downstream rinse section thereof while the welded joint is made, and it has been found that flash rust (stop stain) frequently occurs on the stationary strip in the rinse section. Numerous attempts have been made over the years to eliminate or minimize this problem which manifests itself as orange stains on the steel surface. The quantity and severity of staining does of course vary but may affect a total of up to 50 ft (15 m) of the length of the strip and may result in the whole strip being rejected as unsatisfactory by the customer.